Bolt Stretch Calculator

Bolt Loading

Bolt Stretch Result

Bolt Stretch Breakdown

Bolt Stretch Formula

For a uniform bolt section under an axial tensile load, elastic elongation can be estimated using Hooke's law.

Where ΔL is bolt elongation, F is tensile preload, L is effective bolt length, A is tensile stress area and E is Young's modulus.

Stress and Strain

The calculator first determines the average tensile stress produced by the preload.

Elastic strain is then calculated from stress divided by Young's modulus.

Bolt elongation is the elastic strain multiplied by the effective bolt length.

Effective Bolt Length

The effective length is the portion of the bolt that contributes to elastic elongation in the simplified model. It may differ from the total physical bolt length.

• Include the effective tensile length of the threaded portion where appropriate.
• Account for the geometry of the bolt and joint when determining an engineering-effective length.
• A threaded fastener does not behave exactly like a uniform straight bar.
• For precision preload measurement, use a validated bolt-stretch or ultrasonic elongation procedure.

Example Calculation

For a bolt carrying 50 kN, with a tensile stress area of 245 mm², an effective length of 100 mm and Young's modulus of 200 GPa, the simplified model calculates the elastic elongation directly from the axial-load relationship.

Changing the preload, effective length, stress area or material modulus will change the predicted bolt stretch.

Important Limitations

• This is a simplified elastic bolt-stretch calculator.
• It assumes axial loading and linear elastic material behavior.
• It does not calculate detailed bolt stiffness from complete fastener geometry.
• It does not model thread deformation separately.
• It does not model bearing deformation under the bolt head or nut.
• It does not account for joint compression or joint stiffness.
• It does not calculate preload loss from embedment or relaxation.
• It does not calculate torque-to-preload relationships.
• It does not account for bending, shear or external service loads.
• Actual bolt elongation may differ from the simplified uniform-section calculation.

Frequently Asked Questions

What does the Bolt Stretch Calculator calculate?
It estimates elastic bolt elongation from preload, tensile stress area, effective bolt length and Young's modulus.

What formula is used?
The simplified relationship is ΔL = F × L ÷ (A × E).

Should I use nominal diameter for the area?
For a threaded bolt, tensile stress area is generally more appropriate for a simplified threaded-fastener calculation than the gross nominal cross-sectional area.

What happens if the bolt is longer?
For the same preload, stress area and material modulus, a longer effective bolt length produces proportionally greater elastic elongation.

What happens if the bolt has a larger tensile stress area?
For the same preload, length and material modulus, a larger tensile stress area produces less elastic elongation.

Can bolt stretch be used to determine preload?
Bolt elongation can be related to preload under controlled conditions, but accurate preload determination requires a properly characterized fastener and measurement method.

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